1. Field of the Invention
The invention generally relates to a process for recovering sodium chemicals, including sodium carbonate and/or sodium monohydrate from trona ore formations. More specifically, the invention relates to an improved process for the recovery of sodium based chemicals from sodium containing ore which may be incorporated with traditional monohydrate, solution mining or other sodium extraction processes.
2. State of the Art
In southwestern Wyoming, in the vicinity of Green River, a vast deposit of crude, mineral trona (Na2CO3.NaHCO3.2H2O) which lies some 800 to 3,000 feet beneath the surface of the earth has been discovered. Other such underground deposits of trona have also been discovered in Turkey and China. The Green River trona beds cover 1,000 square miles and consist of several different beds which generally overlap each other and are separated by layers of shale. In some areas, the trona beds occur over a 400 foot stratum with ten or more layers comprising 25% of the total stratum. Several soda ash companies at Green River dry mine a trona seam about 12 feet in thickness at approximately 1,500 feet below the surface, the seam containing about 90% trona. The quality of the trona varies greatly depending on its location in the stratum. A typical analysis of the crude trona being mined at Green River, Wyo., is as follows:
Trona ore is conventionally dry mined by sinking shafts and utilizing miners and machinery to recover the ore from seams below the surface. The underground mining techniques used to recover trona ore vary, including room and pillar mining, continuous mining, long wall mining and other techniques well known in trona mining. Each of these techniques have been employed to improve mining efficiency. However, the cost of dry mining trona ore remains a factor in the production of sodium chemicals, the largest expense resulting from the use of equipment and human labor to recover the trona deposits.
Crude trona ore is processed to convert it into desirable forms such as sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), caustic soda (NaOH), sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O), sodium phosphate (Na5P3O10), or other sodium containing chemicals. One of the most marketable sodium compounds is soda ash, also known as sodium carbonate. Typically, one of two well known processes is used to recover sodium carbonate from trona ore: the Sesquicarbonate process or the Monohydrate process.
The Sesquicarbonate process involves a series of steps, including: dissolving the crude mined trona in a cycling, hot mother liquor containing excess normal sodium carbonate over sodium bicarbonate in order to dissolve the trona congruently, clarifying the insoluble muds from the solution, filtering the solution, passing the filtrate to a series of vacuum crystallizers where water is evaporated and the solution is cooled causing sodium sesquicarbonate to crystallize out of the solution in a stable crystal phase, recycling the mother liquor to dissolve more crude trona and calcining the sesquicarbonate crystals at a temperature sufficient to convert same to sodium carbonate, or soda ash.
A more direct and simplified method developed subsequently to the Sesquicarbonate process is the Monohydrate process. The Monohydrate process tends to yield a dense, organic-free sodium carbonate product through a series of steps which include: calcining the crude trona ore at a temperature of about 125 degrees C. to about 500 degrees C. to convert the trona ore to crude sodium carbonate and to reduce the amount of the organics by oxidation and distillation, dissolving the crude sodium carbonate in water, clarifying the resulting sodium carbonate solution to remove insolubles as muds therefrom, filtering the clarified solution, evaporating water from the clarified and filtered sodium carbonate solution in an evaporator circuit, crystallizing sodium monohydrate crystals from the pregnant mother liquor, calcining the monohydrate crystals to produce a dense, organic-free sodium carbonate product and recycling the mother liquor from the crystals to the evaporation step. The dense soda ash product produced by the Monohydrate process has become the standard product of the trona based sodium carbonate/soda ash industry.
Solution mining has become of more interest as costs associated with dry mining have increased. Furthermore, solution mining offers a process by which the large quantities of ore remaining as pillars, walls, and ceilings in previously dry mined areas may be recovered. In its simplest form, solution mining involves dissolving trona ore into a solution and recovering sodium products from the solution. For example, a trona ore deposit, or other sodium-containing ore, is contacted with a solvent, such as water. The water dissolves the trona ore creating a brine. The brine is recovered and processed to recover sodium carbonate, sodium bicarbonate, sodium sesquicarbonate, sodium monohydrate, or other sodium products. The sodium depleted brine is typically recycled to the trona ore deposit to dissolve additional trona.
One of the downfalls of solution mining, however, is the relatively slow in situ dissolution rate of sodium carbonate and sodium bicarbonate into solution from the trona ore deposits. Typically, solutions formed by the solution mining processes have low concentrations of sodium carbonate and sodium bicarbonate and are therefore lacking the concentrations required to pass the solutions directly into a Monohydrate process or Sesquicarbonate process. Furthermore, solutions recovered from solution mining often vary in concentrations of sodium carbonate and sodium bicarbonate which forces changes in the processing steps and wreaks havoc with process consistency.
The present invention provides a novel process for converting mine water or brines having concentrations of sodium carbonate and/or sodium bicarbonate into sodium containing constituents such as sodium carbonate, sodium bicarbonate, sodium sesquicarbonate, sodium monohydrate, sodium carbonate decahydrate, and the like.
The process involves the neutralization and fortification of brine or mine solutions having sodium carbonate and sodium bicarbonate concentrations. Neutralization, as the term is used herein, is the conversion of a portion of the sodium bicarbonate concentration within the brine or mine solution to sodium carbonate. In the instant process, a brine having concentrations of sodium bicarbonate is neutralized. The neutralized brine is then fortified to increase the concentration of sodium carbonate in the brine or mine solution. Fortification is achieved using calcined trona. The neutralized and fortified brine is suitable for processing or may be added as a supplement to a conventional Monohydrate or other crystallization process.
One example of the process is the recovery of a brine from an underground trona mine. A solvent, such as water, is pumped into an underground trona mine wherein the water dissolves the trona ore. In addition to the water pumped into the trona mine, naturally occurring groundwater may mix with the introduced water. Over time, the water dissolves sodium carbonate and sodium bicarbonate from the trona deposit to form a brine. The brine is pumped from the mine to a processing plant where it is neutralized. Neutralization is usually effected using lime or another caustic which converts at least a portion of the sodium bicarbonate within the brine to sodium carbonate. All of the brine taken from a mine may be treated, or optionally, the brine stream may be split into two streams wherein one stream is neutralized before being re-combined with the second stream to form a neutralized brine stream. In addition to neutralization, the brine stream may be stripped to further convert the sodium bicarbonate to sodium carbonate.
The neutralized brine stream is fortified with calcined trona to increase the total alkalinity of the brine solution. The calcined trona fortification material increases the total alkalinity of the brine by increasing the sodium carbonate concentration without significantly increasing the sodium bicarbonate concentration in the brine. The neutralized and fortified brine is added to a crystallizer where crystals, such as sodium carbonate, sodium monohydrate, and sodium carbonate decahydrate, may be precipitated from the neutralized and fortified brine solution using traditional Monohydrate or other crystallization processes.